The field of the disclosure relates generally to rod pumping units and, more particularly, to a downhole dynamometer and method of operation for computing a downhole dynamometer card for a rod pumping unit.
Most known rod pumping units (also known as surface pumping units) are used in wells to induce fluid flow, for example oil and water. Examples of rod pumping units include, for example, and without limitation, linear pumping units and beam pumping units. Rod pumping units convert rotating motion from a prime mover, e.g., an engine or an electric motor, into reciprocating motion above the wellhead. This motion is in turn used to drive a reciprocating downhole pump located in a production region of the well via connection through a sucker rod string. Generally, the production region of a well is referred to as downhole, at an end of the sucker rod string opposite the wellhead. The sucker rod string, which can extend miles in length down the wellbore to the production region of the well, transmits the reciprocating motion from the wellhead at the surface to a subterranean piston, or plunger, and valves in a fluid bearing zone of the well. The reciprocating motion of the piston valves induces the fluid to flow up the length of the sucker rod string to the wellhead.
The rod pumping units are exposed to a wide range of conditions. These vary by well application, the type and proportions of the pumping unit's linkage mechanism, and the conditions of the well. Furthermore, well conditions, such as downhole pressure, may change over time. These conditions may cause variability in the flow of the fluid.
The rod pumping unit imparts continually varying motion on the sucker rod string. The sucker rod string responds to the varying load conditions from the surface unit, down-hole pump, and surrounding environment by altering its own motion statically and dynamically. The sucker rod string stretches and retracts as it builds the force necessary to move the down-hole pump and fluid. The rod pumping unit, breaking away from the effects of friction and overcoming fluidic resistance and inertia, tends to generate counter-reactive interaction force to the sucker rod string exciting the dynamic modes of the sucker rod string, which causes an oscillatory response. Traveling stress waves from multiple sources interfere with each other along the sucker rod string (some constructively, others destructively) as they traverse its length and reflect load variations back to the rod pumping unit, where they can be measured. Measurements of the position and load of the rod pumping unit at the surface are referred to as a surface dynamometer card, or a surface card.
Generally, the surface measurements are used in diagnostic analysis to determine downhole position and load. The relationship between surface measurements and downhole measurements is represented, for example, and without limitation, by a model of the sucker rod string referred to as the wave equation, which models the propagation of waves in a continuous medium as a one-dimensional partial differential equation. Solutions to the wave equation generally yield a displacement of a point on the sucker rod string at a given time. Translating the surface measurements to downhole measurements by solving the wave equation is computationally intensive and can be inaccurate for highly-deviated wells. The downhole measurements are referred to as a downhole dynamometer card, or pump card.
Downhole dynamometer cards computed using various techniques produce varying results based on conditions at the rod pumping unit and downhole at the pump. Many known rod pumping unit controllers utilize a technique best suited for that particular rod pumping unit. Inaccuracies in the downhole dynamometer card may result in inefficient operation of the rod pumping unit and delayed diagnostic feedback.